The field of the invention is in mass flowmeter instrumentation and particularly in the electronic circuitry art for mass flowmeters.
Mass flowmeters are well known and in wide usage. A typical examples where mass flowmeters are widely used is in the measurement of the fuel flowing into a jet aircraft engine. Mass flowmeters are used to indicate the pounds per hour of fuel flow independent of the absolute values of the density of the fuel and its velocity of flow. In order to fully comprehend this invention it is necessary that the operation of conventional mass flowmeters be understood. Typical prior art devices are schematically represented in FIGS. 1 and 2. The fluid mechanics of the system is illustrated in FIG. 1. The flow sensing means is comprised of two similar rotors 11 and 12 placed coaxially end-to-end, suspended for independent rotation, and separated axially only a small amount occasioned by the stationary disc separator 13. The upstream rotor 11 is commonly referred to as the impeller and the downstream rotor 12 the turbine. Each rotor is comprised of a pair of concentric cylinders with radial vanes dividing the annular space between them into a number of identical flow passages. They are enclosed in a common cylindrical housing in which radial clearances are small enough to prevent appreciable fuel flow around the rotors.
The impeller 11 is driven at constant angular velocity. Each unit mass of fuel (as represented by the small arrows 14) in transit emerges from the impeller flow passages with the angular velocity of the impeller. By virtue of this angular velocity, each unit mass of fuel enters the flow passages of the turbine 12 with angular momentum proportional to impeller speed, but independent of flow rate, fuel density and viscosity, and other ambient conditions. All the angular momentum imparted to the fuel by the impeller is recovered by the turbine so that, in accordance with Newton's Law, the fuel exerts on the turbine 12 a torque directly proportional to the product of mass flow rate and impeller speed.
The turbine 12 is restrained by a spring 15 to deflect through an angle of arc proportional to the torque exerted upon it by the fuel. This angle of rotation, of the turbine 12, moves the pointer 16 across the face of a dial 17 on which the calibration of the corresponding rates of flow are engraved.
FIG. 2 schematically illustrates a conventional control and indicating arrangement as is usually used to drive the impeller and sense and indicate the movement of the turbine; the movement of which is indicative of the mass-flow of the fluids. The fluid flow 21-22 is through the conduit 23 from the fuel source to the item of utilization, typical from a fuel tank to a jet engine. The impeller 11, supported on low friction bearings, is conventionally driven in angular rotation by a two-phase 8 Hz signal in the quadrature coils 24 and 25 which is supplied by the controller 26. These coils are located outside of the fluid conduit 23. The movement of the turbine 12 is sensed by a conventional repeater system (such as a selsyn or synchro type system) having transmitter coil 27 and remote indicator coil 28. The indicator coil 28 conventionally drives the indicator card 29 to an angular position corresponding to the angular movement that the turbine 12 is moved against the restoring force of spring 15 by the mass-flow of the fluid 21-22 passing through the system. The card 29 is conventionally calibrated to indicate the mass fuel flow in pounds-per-hour, or multiples thereof.
Prior to this invention a severe impeller starting problem had existed with aircraft having low primary voltage during the startup cycle or at any time the nominal 115 v 400 Hz primary voltage dropped to less than about 80 volts and then was slow to regain a nominal 115 volt value. One of the reasons for difficulty in starting the impeller with low primary voltage is due to the eddy currents in the aluminum conduit or housing 23 separating the two-phase starter windings 24 and 25 from the permanent magnet rotor in the impeller. These eddy current losses can be reduced by using a titanium or other higher resistance housing, but of a great expense. Another way that has been attempted to make the impeller easier to start has been to incorporate swirl vanes to provide a flow assist effect but swirl vanes cause an increased pressure drop and in most applications, particularly military, an extremely low pressure drop is a requirement that is placed on the mass flowmeter transmitter so that maximum fuel flow with minimum pump energy is obtained. Obviously providing sufficient power at 8 Hz to start under all conditions of temperature, low voltage, and high flow is impractical because of indicator power limitations and the cost of the required higher rated electronic components. Thus, it was highly desirable to discover some means of starting the two-phase synchronous 8 Hz impeller at a lower voltage than its nominal designed voltage. It has been found that once the impeller is in rotational motion, it will lock into synchronism and properly function at voltages of about 80 volts and above, but that it is normally incapable of starting at voltages approximately equal to or less than 80 volts. It is during the startup period when voltages are low that it is also very important to know the fuel flow into the engine.
Typical examples of the prior art in connection with the starting of synchronous motors are exemplified by U.S. Pat. Nos. 3,408,547 to patentee W. Saeger; 3,855,510 to patentee D. J. Houck; 3,219,897 to patentee A. Beltrami and 3,582,735 to patentee A. P. Maruschak.